1. Field of the Invention
The present invention relates to aqueous polyurethane (PU) dispersions. More particularly, it relates to particulate aqueous polyurethane dispersions having the core-shell morphology and the process for producing the same.
2. Description of the Related Arts
Polyurethane is one of five widely-used resins and also is one of three highly-functional resins. However, over 90 percent of polyurethanes contain quite a lot of organic solvent such as N,N-dimethylformamide or toluene, which pollutes the environment and endangers the health of operators. Since environment protection is gaining world-wide attention, and pollution laws are becoming stricter, the polyurethane resin industry has made revolutionary progress in recent years by using low-polluting aqueous polyurethane resins instead of high-polluting, solvent type polyurethane resins.
In the early 1960s, many patents with respect to aqueous polyurethanes appeared. In the early stages, the patents with respect to aqueous polyurethane focused mainly on development and research for manufacturing, while in recent years, they have focused mainly on improving the physical properties of polyurethanes, new applications, and improvement of manufacturing process. For example, EP 684286 discloses an aqueous polyurethane dispersion suitable for use in adhesives or surface coating, which will coagulate when being heated. JP 07082333 discloses the use of polyamine as a chain extender to obtain an aqueous polyurethane with superior film heat resistance. JP 07102030 discloses a radiation-curable aqueous polyurethane. However, most of the aqueous polyurethanes disclosed in these patents are still in the experimental stage, and their physical properties such as tensile strength, elongation, abrasion resistance, and solvent resistance have yet to meet commercial requirements.
A conventional process for producing aqueous polyurethane resins includes prepolymerizing a polyol, a hydrophilic group-containing dihydric alcohol, and a diisocyanate in a high-boiling-point organic solvent; neutralizing the prepolymer with a tertiary amine to ionize the hydrophilic group; dispersing the neutralized prepolymer in water; and finally chain-extending the dispersed prepolymer to obtain aqueous polyurethane dispersions.
However, in the conventional process for producing an aqueous polyurethane, part of the terminal isocyanate groups of the prepolymer will be consumed by water upon dispersing and converted into amino groups. Consequently, the isocyanate groups cannot effectively react with a chain extender, a diamine for example, to extend the chains and raise the molecular weight, thereby adversely affecting the physical properties of resulting polyurethanes.